Fashioned for Flight.
The envelope of air that surrounds our planet is essential for all known forms of life. Yet, as a medium for creatures to move in, it poses an enormous challenge, for it offers neither the support of terra firma nor the buoyancy of water. It is not surprising, therefore, that over the long course of history, only a few vertebrate groups ever achieved the ability to fly: the long-extinct pterodactyls, bats, and birds.
No doubt, birds attract our attention because of their widespread occurrence, great variety, colorful plumage, and characteristic calls. But for many of us, their most captivating feature is their mastery of flight. We marvel at their ability to soar and dive, glide and turn, take off and land--all done with seemingly little effort, yet often involving great precision. How do they do it? Which of their secrets have helped us build flying machines?
The aerodynamic design of birds is dictated by three factors that affect flight: lift, thrust, and drag. Lift is the upward force that overcomes the pull of gravity, thrust corresponds to the propulsive force for forward movement, while drag is the air friction against the bird's body. The feathers' contours and the body's teardrop shape help reduce drag, but lift and thrust are generated by the wings. For an aircraft, the wings likewise supply lift, but the plane requires engines to thrust it through the air.
The wings of birds and airplanes are cambered airfoils, with a thicker leading edge and a thinner trailing edge. If viewed in cross section, the upper wing surface appears convex, while the lower surface is flat or somewhat concave. When the wing moves, the air flows faster over its top than along its underside, so the air pressure above is less than that below. This pressure difference creates the lift that counteracts gravity and helps launch the bird and maintain it in the sky.
A bird's wings are made of feathers, skin, bones, and muscles. The bone structure of a wing resembles that of a human forelimb, consisting of an upper arm, forearm, and hand. Attached to the wing bones are three types of flight feathers. The primary flight feathers (or primaries) are attached to the hand bones; the secondaries are connected to the forearm; and the tertiaries are on the upper arm. In addition, the base of the flight feathers is covered by covert feathers, on the wing's upper and lower surfaces. The feathers give the wing its streamlined shape and are vital for propulsion, control, and lift.
For most birds, the flapping of wings involves a complex pattern. The wingbeat begins with a power stroke (also called the downstroke), in which the bird pulls its wings forward and downward, creating lift over the wing. At the bottom of the power stroke, the wing's distal end twists backward to push against the air and generate thrust, comparable to a swimmer pushing his hands back against the water to swim forward. During the recovery stroke (or upstroke), the wings tuck inward and then move forward again in preparation for the next power stroke. Throughout the downstroke, the flight feathers closely overlap to enhance lift and thrust; but during the upstroke, they slot open like venetian blinds to permit airflow between the feathers and reduce drag.
Additional lift can be gained by tilting the wing's leading edge upward into the airstream. As the upward tilt increases the angle of attack, the speed of air flowing over the top of the wing also increases, thereby generating more lift and thrust. But if the angle of attack exceeds about 15 degrees, airflow breaks away from the upper wing into turbulent eddies, reducing lift and causing the wing to stall.
To delay turbulence and maintain stability while slowing down, the bird separates some of its primary feathers, forming wing slots. The slotting provides an even flow of air over the upper wing surface. Aircraft use flaps and spoilers to control airflow over the wing, but birds must slot their flight feathers to accomplish the same purpose. The widely spaced primary flight feathers of soaring hawks and gliding vultures offer a clear example of wing slotting.
Most birds also have a small cluster of three or four feathers at the wing elbow. This cluster, called the alula, slots open (projects upward) to create a small airfoil just above and in front of the wing. This airfoil directs a smooth flow of air over the wing, allowing the bird to maintain lift and stability at the slower speeds associated with landing or takeoff.
The tail serves various functions in flight. It acts as a stabilizer and rudder when flying and a brake when slowing down and landing. Also, many birds add lift by spreading their tail feathers wide when flying. In some species, the tail assumes a courtship function, being either greatly elongated or elaborately adorned to attract the opposite sex. The long tails of male pheasants and peacocks, for instance, intrigue females but impede flight.
Appropriately sculpted body
Wings alone are insufficient for flight. The entire body must be appropriately sculpted to achieve maximum strength and lightness. In birds, these features govern the structures and functions of the feathers, skeleton, muscles, and internal systems.
Feathers are the most distinctive and unique characteristic of birds [see sidebar]. They are lightweight, yet combine tensile strength with tough resilience. As a result, they are perfect for the surfaces of the wings and tail and other stress points during flight.
Like the pterodactyls of an earlier era and unlike mammals today, birds have bones that are thin and hollow but reinforced with internal struts for support. Trunk vertebrae are fused, and the ribs are connected by transverse struts that minimize skeletal compression and distortion during flight. Even the tailbones are a simple nubbin that barely supports the tail feathers. To compensate for this rigid skeleton, birds have twice as many neck vertebrae as most mammals, allowing maximum flexibility of the head and neck.
While all else is minimized, the flight muscles are greatly enlarged. By far the largest of these muscles are the pectorals (in the breast), which pull the wing forward and down during the power stroke. These muscles account for nearly a quarter or more of the weight of hummingbirds and other strong fliers. To facilitate oxygen delivery to flight muscles, birds have the most efficient respiratory and circulatory systems in the animal kingdom. In most cases, the breathing rate and heartbeat are much faster than for other animals.
The wings of all birds have the same basic framework, but birds of different habits and habitats have distinctively different wing shapes and sizes, providing appropriate combinations of speed, lift, and maneuverability. For instance, swallows, swifts, and falcons have thin, narrow, swept-back wings, allowing them to achieve high-speed flight. The narrow sweep reduces air friction across the wing surface, while the tapered wingtips eliminate wingtip vortices that begin at higher speeds.
The champion speedster is the peregrine falcon, which dives on prey at speeds approaching 200 MPH. So too, narrow wings promote the seemingly effortless, long-distance migrations of sandpipers and plovers, which can fly in excess of 100 MPH, hour after hour, as they migrate over the world's oceans.
The low drag of narrow wings is also an advantage for soaring seabirds, such as albatrosses and gannets. Albatrosses use their extraordinarily long wings to exploit the light but steady winds just above the waves in the midlatitude regions of the world's oceans. The wings are highly effective in providing lift, but their lack of slotting severely restricts the bird's stability at slow speeds. Consequently, albatrosses are noted for their ungainly takeoffs and clumsy landings, earning them the nickname "gooney" birds.
By contrast, eagles, hawks, and vultures have broad wings that serve as stable platforms, enabling them to soar for hours in search of prey. The distinctive slots between the primary feathers facilitate slower flight, which is especially useful when the raptor takes off while clutching prey in its talons. The broad wings of these birds can extract maximum lift from air currents.
Vultures typically have the weakest flight muscles of all birds, so they find and soar in bubbles of warmer air--called thermals--as they rise in great circles over the landscape. Smaller vulture species take to the air in the relatively weak thermals of the early morning, but larger species await the greater warmth of later hours to create stronger thermals. The largest vultures and heavy-bodied storks fly only during the hottest hours of the day, when the strongest thermals are available for support. Once they are airborne, however, their longer wings enable them to glide for great distances. This is why American condors and African vultures can travel far in their daily search for food.
Birds that dwell in woodlands and other complex habitats typically have elliptical or rounded wings, with distinctively slotted wingtips. They include, for example, woodpeckers, warblers, doves, and magpies. Their rounded wings are best adapted for quick changes of direction. They also provide the extra lift needed for the slower flights and frequent takeoffs and landings involved in flitting through vertically structured habitats. Their primary feathers are widely slotted, and each of these feathers can act as a separate wing, contributing to the lift and thrust needed to maneuver around vegetation and other structures at slower speeds.
Wings support weight, so bigger birds must have larger wings to provide the necessary lift for flight. But if the bird is too heavy bodied, its weight cannot be supported by the wings. Today's large, flightless birds--ostriches, cassowaries, rheas, and emus--are descendants of flying birds, but they are far too heavy to be supported by their rudimentary wings and are permanently grounded. The 300-pound ostrich, for example, would require a wingspan of nearly 200 feet to fly-- clearly impossible to power with muscles and support with flesh and bone.
Exactly how flight originated continues to be controversial. Of the many theories advanced, the two most plausible are labeled "from the ground up" and "from the trees down." John Ostrom of Yale University is a leading proponent of the theory that ancestral birds learned to fly from the ground up. Citing Archaeopteryx as an example, he proposed that the ancestors of birds were speedy, long-legged predators that used feathery wingtips to trap insects. Over time, their feather-coated forelimbs gradually acquired adaptations to become incipient wings for longer and longer glides, followed by sustained flights.
Other scientists, however, argue that wings would slow a running predator instead of enhancing its speed. They believe that the ancestors of birds learned to fly by climbing into treetops and gliding from tree to tree or from trees to the ground, gradually transitioning from gliding to true flight.
If the ancestral bird had four perfectly good legs, why did it bother to learn to fly? We may never know all the answers, but the ability to fly opens enormous opportunities. The flying animal can explore, discover, and colonize new habitats. It can capture aerial insects and have better access to fruits, nuts, and berries before they spoil. Flight also provides a quick and convenient means of escape from predators.
It is no accident that in the nearly 200 million years of their existence, birds have come to occupy and exploit virtually every habitat, from the tropics to the poles and even the remotest islands of the world. Flight makes possible the seasonal movements and migrations by which birds exploit favorable habitats throughout the year. Clearly, their absolute mastery of flight has placed birds among the most widespread, successful, and fascinating of all animals.n
Dwight G. Smith is professor and chairman of the biology department of Southern Connecticut State University in New Haven. His most recent book, Animal Biology and Biodiversity, will be released this spring by Pearson Publishing.
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|Author:||Smith, Dwight G.|
|Publication:||World and I|
|Date:||Mar 1, 2001|
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